The transparent conductive films have a high conductivity (for example, a resistivity of 1×10−3 Ωcm or less) and a high transmittance in the visible region and thus are utilized as the electrodes of solar cells, liquid crystal display elements, and other various light-receiving elements, followed by heat wave reflective films used for window glass of cars and buildings, various antistatic films, and antifogging transparent heating elements for refrigerating show cases.
For the transparent conductive films, antimony- or fluorine-doped tin oxide (SnO2) films, aluminum- or gallium-doped zinc oxide (ZnO) films, and tin-doped indium oxide (In2O3) films are widely utilized. In particular, the tin-doped indium oxide films, namely In2O3-Sn based films, are referred to as ITO (indium tin oxide) films and are most main current materials widely used in various devices, not to speak of LCDs, because low-resistance transparent conductive films are easily available. When the ITO film is deposited on the substrate by a sputtering process at room temperature, a conductive film with a thickness of 200 nm and a surface resistance of about 25Ω/□ (about 5×10−4 Ωcm in terms of a resistivity) is obtained.
On the other hand, a transparent conductive film different from a simple ITO film, namely a transparent conductive film formed by the lamination of transparent oxide thin films and a metallic thin film is proposed. In Patent Reference 1 described below, for example, a transparent conductive film having a three-layer structure that a silver-based alloy thin film of 5-20 nm in thickness is sandwiched between transparent oxide thin films is disclosed and has the feature that each of the transparent oxide thin films is an oxide mixing a first base material containing at least one kind of oxide of a metal that is easy to dissolve with silver and a second base material containing at least one kind of oxide of a metal that is hard to dissolve with silver and the silver-based alloy thin film is a silver alloy containing gold at a minimum. It is further disclosed that the first base material included in the transparent oxide thin film is indium and the second base material is cerium (an In—Ce—O film, which is sometimes described as an ICO film).
Generally, the surface resistance of the ITO film with a thickness in the order of 100 nm, deposited at room temperature, is nearly 50Ω/□. In contrast to this, the surface resistance of the above laminated film with a thickness of 50-100 nm can be reduced to 10Ω/□ or less, depending on the thickness of the silver-based alloy thin film, and to 5Ω/□ or less, when occasion demands.
Recently, luminescent materials and light-emitting devices (for example, LEDs, lasers, and organic or inorganic EL elements) having the functions of blue light emission and near-ultraviolet emission (for example, with wavelengths of 300-400 nm) and solar cells converting solar radiation into electric power have been widely popularized. (For near-ultraviolet LEDs, refer to Non-Patent References 1 and 2 described below.) The transparent electrodes are also indispensable for these electronic devices.    Patent Reference 1: Japanese Patent Kokai No. Hei 9-176837    Patent Reference 2: Japanese Patent Kokai No. Hei 7-182924    Patent Reference 3: Japanese Patent Kokai No. Hei 9-259640    Non-Patent Reference 1: Appl. Phys., Vol. 68 (1999), No. 2, pp. 152-155    Non-Patent Reference 2: SEI Technical Review, September 2004 (No. 165), pp. 75-78